Aren't the same conditions true for chess, though there is no proof that a best strategy exists?
What if it's not just rock/paper/scissors, and there are more than 3 strategies?
What if it's not just rock/paper/scissors, and there are more than 3 strategies?
If there are more than 3 strategies, the same argument stands. IF there are multiple "optimal" strategies, AND these strategies create different "optimal" moves on the same board state, the opponent can just play the optimal counter-strategy after you make your move, because your move is enough information to give away your strategy. If your opponent cannot determine your strategy after you make your move, that implies the different strategies have the same "optimal" move.